The momenttomoment control of movement depends both upon the descending commands to the spinal cord and on activity in spinal reflex pathways. Although the reflex actions of muscle stretch have been extensively studied, much less is known about the reflex actions of muscle stretch have been extensively studied, much less in known about the reflex actions of muscle contraction. In particular, there has been very little work on the neuronal mechanisms and functional role of reflex effects directed from the contracting muscle to other muscles. The purpose of the proposed research is to provide a quantitative analysis of these contractionevoked reflexes and to investigate their afferent and interneuronal mechanisms. This general goal can be divided into 3 specific aims: 1. To describe the pattern of reflex effects produced by muscle contraction in the decerebrate cat (with or without an additional partial spinal section). Reflex effects produced by isometric contractions (via ventral root stimulation), stretch, vibration and electrical stimulation of muscle nerves will be compared with the responses of different types of muscle afferents to determine the afferent source of these effects. 2. To describe the response of Group 1 and force activated interneurons to muscle contraction, stretch, vibration and nerve stimulation. Quantitative comparisons of reflex effects with afferent and interneuron response will be used to identify those interneurons most likely to mediate contractionevoked reflexes. 3. To determine the specific output connections of Group 1 and force activated interneurons by spiketriggered averaging from neuron discharge into electomyographic activity in various extensor muscles.